We request support for a fourth continuation of a previously funded project lasting 10+ years. The proposed work involves a broad study of genetic and environmental influences on fertility outcomes and the process leading up to them. Motivation for the project derives from past work studying social influences on adolescent transition behaviors, including sexuality and pregnancy, smoking, drinking, delinquency, intellectual development, and achievement. Recently, we have begun to use models from developmental psychology and behavior genetics that allow us to study genetic influences along with social/environmental influences on behaviors. Our previous work has used the National Longitudinal Survey of Youth extensively; however, the NLSY does not contain explicit information distinguishing full, half, and adoptive siblings. We have developed a "linking algorithm" to define these kinship links. In addition, we have used an analysis approach--an adaptive DF Analysis model--that partitions variance into genetic, shared environmental, and non-shared environmental influences. Our most recent work has studied genetic and environmental influences on age at first intercourse. The current application extends that study into a large framework and considers age at menarche, age at first intercourse, age at marriage, age at first pregnancy, age at first birth, age at subsequent births, lags between these various outcomes, and a set of attitudinal variables measuring fertility desires and expectations. These variables are defined and organized with the assistance of the Life Course Perspective. We propose four sub-studies: In each study we will: (a) identify whether genetic influences act on the various fertility behaviors; (b) evaluate whether shared environmental influences are important; (c) statistically test for the importance of several specific and well-chosen non-shared environmental influences; (d) evaluate whether extreme versions of fertility behavior have the same etiology as the broadly defined behaviors; (e) identify race and gender differences in the patterns we find.